Anyway, after rediscovering my original 520ST with external power brick I cleaned it all up and hooked it up to a mono monitor from a MegaST4. It ran for long enough for me to start cleaning the heads on the SF354 floppy then I watched the screen flicker, fade and eventually die. Suspecting the monitor since it hadn't been powered up for $years I dug a similarly old SM124 out of the garage and got similar amounts of nothing. It wasn't till then I noticed the power light on the ST itself hadn't actually come back on so I switched over supplies from the floppy drive and the machine sprang into life again.

I've managed to get the power brick apart and it's a nasty looking NORTEL board run by an NE5561. All the caps test ok with a DMM but seem to defeat my basic ESR meter, I suspect it can't deal with 4700uF caps.

Is this worth repairing? It's a simple +5@3A and +12/-12@30ma brick and I've got an old Cisco PIX supply that fits the bill perfectly. I did plug it into the floppy drive and the motor spins so it's only the 5V rail that's gone but helpful stuff like schematics don't seem to exist online.